Natural dissolved organic matter (DOM) in aquatic systems plays many environmental roles: providing building blocks and energy for aquatic biota, acting as a sunscreen in surface water, and interacting with anthropogenic compounds to affect their ultimate fate in the environment. Such interactions are a function of DOM composition, which is difficult to ascertain due to its heterogeneity and the co-occurring matrix effects in most aquatic samples. This review focuses on current approaches to the chemical structural characterization of DOM, ranging from those applicable to bulk samples and in situ analyses (UV-visible spectrophotometry and fluorescence spectroscopy) through the concentration/isolation of DOM followed by the application of one or more analytical techniques, to the detailed separation and analysis of individual compounds or compound classes. Also provided is a brief overview of the main techniques used to characterize isolated DOM: mass spectrometry (MS), nuclear magnetic resonance mass spectrometry (NMR) and Fourier transform infrared spectroscopy (FTIR).

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Dissolved organic matter (DOM) is a complex mixture of molecules found ubiquitously in freshwater and saltwater environments. Its structures contain valuable information content on the sources of molecules as well as the mechanisms at work within an aquatic ecosystem. Recent advancements in high resolution mass spectrometry and liquid chromatography have made inroads into determinations of the molecular structures within DOM. Such analyses, however, generally require a prior step to concentrate/isolate DOM, and this step often limits the number of samples that can be analyzed. This study has developed a fast method to concentrate DOM on commercially available online solid phase extraction (SPE) cartridges, which can be directly eluted onto an HPLC‐MS system. This rapid solid phase extraction (RSPE) method requires less sample (10‐100 mL) than previous SPE methods for DOM isolation. Additionally, this study tested a suite of SPE phases to find a combination that improves DOM recovery as compared with commonly used approaches. When a polystyrene divinylbenzene phase (RP‐1) was coupled with an activated carbon (CAR) phase, recoveries were found to be significantly higher than in previous SPE studies relying upon single phases. RSPE was tested for a diverse set of salty and freshwater samples with recoveries ranging from 46% to 78% of the total DOC. It was also tested on a suite of model compounds (including caffeine) and should be applicable to anthropogenic compounds in aquatic environments, although, in such cases, optimization may be needed to minimize the natural organic matter signal that was maximized in this study.

Abstract. The net ecosystem exchange (NEE) and methane (CH4) flux were measured by chamber measurements for five distinct ecotypes (areas with unique eco-hydrological characteristics) at Abbeyleix Bog in the Irish midlands over a 2-year period. The ecotypes ranged from those with high-quality peat-forming vegetation to communities indicative of degraded, drained conditions. Three of these ecotypes were located in an area where peat was extracted by hand and then abandoned and left to revegetate naturally at least 50 years prior to the start of the study. Two of the ecotypes were located on an adjacent raised bog, which although never mined for peat, was impacted by shallow drainage and then restored (by drain blocking) 6 years prior to the start of the study. Other major aspects of the carbon (C) balance, including dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and open-water CO2 evasion, were quantified for a catchment area at the study site over the same 2-year period. The ecotype average annual ecotype C balance ranged from a net C sink of -58±60 g C m−2 yr−1, comparable to studies of intact peatlands, to a substantial C source of +205±80 g C m−2 yr−1, with NEE being the most variable component of the C balance among the five ecotypes. Ecotype annual CH4 flux ranged from 2.7±1.4 g C-CH4 m−2 yr−1 to 14.2±4.8 g C-CH4 m−2 yr−1. Average annual aquatic C losses were 14.4 g C m−2 yr−1 with DOC, DIC, and CO2 evasion of 10.4 g C m−2 yr−1, 1.3 g C m−2 yr−1, and 2.7 g C m−2 yr−1, respectively. A statistically significant negative correlation was found between the mean annual water table (MAWT) and the plot-scale NEE but not the global warming potential (GWP). However, a significant negative correlation was observed between the plot-scale percentage of Sphagnum moss cover and the GWP, highlighting the importance of regenerating this keystone genus as a climate change mitigation strategy in peatland restoration. The data from this study were then compared to the rapidly growing number of peatland C balance studies across boreal and temperate regions. The trend in NEE and CH4 flux with respect to MAWT was compared for the five ecotypes in this study and literature data from degraded/restored/recovering peatlands, intact peatlands, and bare peat sites.

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